This invention relates generally to thermostatic electrical switches and more particularly to such switches having a creep acting thermostatic member.
Thermostatic electrical switches using a heat responsive thermostatic switching element comprising an element formed of metal layers having different coefficients of thermal expansion, such as a bimetal element, are either snap-acting or creep acting. In snap-acting switches the thermostatic element is formed into a dished configuration which will change from one dished configuration, e.g., concave, to an opposite dished configuration, e.g., convex, upon selected changes in temperature. An example of this type of switch is shown in U.S. Pat. No. 3,430,177. Switches of this type, comprising a cantilever mounted snap-acting member having a movable contact attached to its free distal end and movable into and out of engagement with a stationary contact, are used to protect motors, generators, transformers and other electrical apparatus from overheating caused by excessive currents or other temperature conditions. In order to closely monitor the temperature of the apparatus, the switches are placed within the apparatus and are therefore miniaturized in order to be used with miniaturized motors, generators, transformers and the like. Switches of the type shown in the above noted patent are widely used with many millions of such switches in use.
In creep acting switches, the thermostatic element is formed into a generally flat plate which bends as its temperature changes. A general guideline in the design of cantilever mounted creep thermostatic elements is that a 2:1 length to width ratio is required for good, predictable creep action of the thermostatic element. Creep action thermostatic elements made according to the guideline generally require a larger housing than is required for the snap-acting elements described above. Creep acting switches are used, in small appliances, such as heating pads, to maintain a generally constant temperature.
It is an object of the present invention to provide a creep acting thermostatic element made in accordance with the recommended length to width ratio and at the same time be sized so that it can be used in a package size comparable to that used for snap-acting thermostatic members. Another object of the invention is the provision of a miniaturized thermostatic switch having a creep acting, cantilever mounted thermostatic switch having a creep acting, cantilever mounted thermostatic element which can be mounted in a miniaturized housing generally just large enough for snap-acting members. Yet another object is the provision of a creep type thermostatic switch and switching element which is free of the prior art limitations noted above.
Briefly, in accordance with the invention, a creep acting thermostatic element comprises a generally flat blade of suitable thermostatic material such as bimetal which has been permanently deformed into a generally J-shaped configuration. This arrangement allows for an effective length almost the same length as the housing. The J-configuration provides a means for using a wider blade in a smaller package thereby increasing disc forces for a given size package which in turn increases cycle life. According to a feature of the invention, a relatively flat portion of the blade is formed between the two legs and is used to avoid interference with the corner formed between the end wall and the bottom wall of the housing.
Other objects and features of the invention will become more readily understood from the following detailed description and appended claims, when read in conjunction with the accompanying drawings in which like reference characteristic""s designate like parts throughout the FIGURES thereof.